Synthetic and biochemical studies will be continued of the mechanisms of several steps in the complement cascade, the major humoral immune system for host defense. (1) Peptide lactams and thiolactones based on two structures proposed for the metastable binding sites of complement proteins C3b and C4b and the serum protease inhibitor Alpha2-macroglobulin will be synthesized. Kinetic studies of the hydrolysis, aminolysis, alcoholysis and hydride reduction of these peptide models will be carried out to obtain indirect evidence for the structure(s) of the metastable binding sites in these serum proteins. Model studies will be conducted on the role of the side-chain carboxylate group of the glutamic acid residue in the reactivity of the metastable binding sites with nucleophiles. Model studies will also be performed on the role of the side-chain imidazole ring of the histidine residue preceding the metastable binding site of C3b in the high reactivity of this site. Model studies will be undertaken of involvement of a transglutaminase in converting a glutamine residue into an internal pyroglutamyl residue during biosynthesis of the metastable binding site. (2) Peptides containing residues from both terminii of human C5a anaphylotoxin will be synthesized. These C5a peptides will be bioassayed as agonists and antagonists of the spasmogenic, chemotactic, and immunoregulatory activities of native C5a. If necessary to identify the minimum active site, most or all of the 75-residue chain of C5a will be synthesized. (3) Active-site peptides from C3a anaphylatoxin having 2-(carboxymethyl)-arginine at the COOH terminus will be synthesized and bioassayed as inhibitors of the C3a/C5a regulatory enzyme carboxypeptidase N and as long-lived spasmogens. (4) Synthetic peptides containing multiple copies of an IgG segment that inhibits C1q binding will be bioassayed as inhibitors or activators of C1-mediated immune hemolysis. These mechanistic studies are relevant to possible new modes of therapy for autoimmune diseases, such as rheumatoid arthritis, systemic lupus erythematosis, and certain glomerular nephritides.